The invention relates to systems and methods for mapping the interior regions of the heart for treatment of cardiac conditions.
Physicians examine the propagation of electrical impulses in heart tissue to locate aberrant conductive pathways. The aberrant conductive pathways constitute peculiar and life threatening patterns, called dysrhythmias. The techniques used to analyze these pathways, commonly called xe2x80x9cmapping,xe2x80x9d identify regions in the heart tissue, called foci, which are ablated to treat the dysrhythmia.
Conventional cardiac tissue mapping techniques use multiple electrodes positioned in contact with epicardial heart tissue to obtain multiple electrograms. Digital signal processing algorithms convert the electrogram morphologies into isochronal displays, which depict the propagation of electrical impulses in heart tissue over time. These conventional mapping techniques require invasive open heart surgical techniques to position the electrodes on the epicardial surface of the heart.
Furthermore, conventional epicardial electrogram processing techniques used for detecting local electrical events in heart tissue are often unable to interpret electrograms with multiple morphologies. Such electrograms are encountered, for example, when mapping a heart undergoing ventricular tachycardia (VT). For this and other reasons, consistently high correct foci identification rates (CIR) cannot be achieved with current multi-electrode mapping technologies.
Researchers have taken epicardial measurements of the electrical resistivity of heart tissue. Their research indicates that the electrical resistivity of infarcted heart tissue is about one-half that of healthy heart tissue. Their research also indicates that ischemic tissue occupying the border zone between infarcted tissue and healthy tissue has an electrical resistivity that is about two-thirds that of healthy heart tissue. See, e.g., Fallert et al., xe2x80x9cMyocardial Electrical Impedance Mapping of Ischemic Sheep Hearts and Healing Aneurysms,xe2x80x9d Circulation, Vol. 87, No. 1, January 1993, 199-207.
This observed physiological phenomenon, when coupled with effective, non-intrusive measurement techniques, can lead to cardiac mapping systems and procedures with a CIR better than conventional mapping technologies.
A principal objective of the invention is to provide improved probes and methodologies to examine heart tissue morphology quickly, accurately, and in a relatively non-invasive manner.
One aspect of the invention provides systems and methods for examining heart tissue morphology using three or more spaced apart electrodes, at least two of which are located within the heart in contact with endocardial tissue. The systems and methods transmit electrical current through a region of heart tissue lying between selected pairs of the electrodes, at least one of the electrodes in each pair being located within the heart. Based upon these current transmissions, the systems and methods derive the electrical characteristic of tissue lying between the electrode pairs.
This electrical characteristic (called the xe2x80x9cE-Characteristicxe2x80x9d) can be directly correlated to tissue morphology. A low relative E-Characteristic indicates infarcted heart tissue, while a high relative E-Characteristic indicates healthy heart tissue. Intermediate E-Characteristic values indicate the border of ischemic tissue between infarcted and healthy tissue.
According to this aspect of the invention, the systems and methods derive the tissue E-Characteristic of at least two different tissue sites within the heart without altering the respective positions of the endocardial electrodes. The systems and methods make possible the differentiation of regions of low relative E-Characteristic from regions of high relative E-Characteristic, without invasive surgical techniques.
Another aspect of the invention provides systems and methods that generate a display showing the derived E-Characteristic in spatial relation to the location of the examined tissue regions. This aspect of the invention makes possible the mapping of the E-Characteristic of heart tissue to aid in the identification of possible tissue ablation sites.
How the E-Characteristic is expressed depends upon how the electrical current is transmitted by the electrode pair through the heart tissue.
When one of the electrodes in the pair comprises an indifferent electrode located outside the heart (i.e., a unipolar arrangement), the E-Characteristic is expressed in terms of tissue impedance (in ohms). When both electrodes in the pair are located inside the heart (i.e., a bipolar arrangement), the E-Characteristic is expressed in terms of tissue resistivity (in ohmxc2x7cm).
In a preferred embodiment, the systems and methods employ electrodes carried by catheters for introduction into contact with endocardial tissue through a selected vein or artery. The systems and methods transmit electric current and process information through signal wires carried by the electrodes. The electrodes can be connected to a multiplexer/demultiplexer element, at least a portion of which is carried by the catheter, to reduce the number of signal wires the catheter carries, and to improve the signal-to-noise ratio of the data acquisition system.
Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended claims.